You have 120.0 mL of a 0.245 M solution of NaCl in an open beaker. After several days, you
test the solution and find that it now has a concentration of 0.348 M. How much water must
have evaporated from the beaker given the new concentration?

So how many mols did you have to start? That's mols = M x L = 0.120 x 0.245 = 0.0294.
When some of the water has evaporated you know you still have 0.0294 mols there. And the molarity now is 0.348M and not 0.245M. So M = mols/L. M = 0.348; mols = 0.0294. Solve for L of solution and subtract from the starting volume to find how much has evaporated.